Solar cells normally have very shallow p-n junctions so that they may capture the blue end of the photo spectrum. Typically, the p-n junction lies approximately 0.3 micrometers beneath the surface of the semiconductor wafer.
In forming the junction, the semiconductor wafer is exposed at appropriate temperatures to a source of doping material selected to provide the desired p-n junction.
For example, in a boron doped, silicon semiconductor wafer which is a p-conductivity type wafer, a layer of phosphorous pentoxide glass is deposited over the entire surface of the wafer through chemical vapor deposition by flowing phosphorous oxychloride and oxygen into a diffusion chamber using a carrier gas of dry nitrogen. The interreaction of the silicon wafer with these gases and the formation of the phosphorous penetoxide glass provides sufficient phosphorous atoms to diffuse from the glass into the silicon surface and thereby provide the desired p-n junction.
To form the operative solar cell, the layer of phosphorous pentoxide glass is usually, but not necessarily, removed and the diffused layer of silicon removed from the silicon wafer except for one surface thereof so that appropriate electrical contacts can be attached to the front and back of the wafer. The undesired portions of the phosphorous pentoxide glass layer can be removed by acid etching, plasma etching, and the like.
Thereafter the front and back electrical contacts, to which wires or other conductors are ultimately to be soldered, are placed on the front and back of the wafer by various known methods such as contact metallization.
Thereafter the electrical contact-bearing wafer is usually washed with concentrated hydrofluoric acid to reduce the series resistance of the contacts and to render the electrical contacts on the wafer more solderable. However, this washing has been found sometimes to leave acid residue on the wafer, which residue is moisture sensitive. It is highly desirable to keep the wafer essentially moisture free in the final solar cell assembly. Therefore, it is desirable to minimize or eliminate such a residue.